In a dry etching process of aluminum, decomposed resist material supplied from resist contributes to a formation of sidewall protection film which is known to permit anisotropic micro-processing. Chlorine-based gases such as Cl2, BCl3, and HCL are typical for the etching. Aluminum, which is a highly reactive metal, reacts vigorously with Cl2 molecules and is etched rapidly. The formation of protection film on the sidewall of a pattern is thus necessary in order to suppress undercut. A fine pattern with a high resist opening ratio, however, causes lack of the decomposed resist material to form sufficient sidewall protection films, leading to occurrence of undercut during the etching, which results in failure in maintaining the anisotropy of the etched aluminum. Consequently, decrease in interconnection reliability and breaking of interconnection may occur.
A countermeasure for the drawback is known, as disclosed in Patent Document 1, to form a dummy resist pattern separately from a device pattern on the same wafer, whereby the increased resist amount supplements the decomposed resist material and keeps the anisotropy of the aluminum. Another method is known, as disclosed in Patent Document 2, to introduce an impurity that forms a sidewall protection film in aluminum etching, for example, element boron, into an aluminum film in advance.    Patent Document 1 JP 63-250823 A    Patent Document 2 JP 07-45590 A
Resist opening ratio can be high in some semiconductor devices or ICs because of their characteristics. Besides, further shrinking of a chip size is expected to reduce the degree of freedom in interconnection layout and to make it difficult to form a large dummy pattern covered with resist. A large dummy pattern can only be formed at a location apart from the interconnection pattern, causing a problem in which the decomposed resist material may exert little effect to the formation of the sidewall protection film. The micro-loading effect also raises a problem in that the decomposed resist material cannot be supplied to a fine interconnection pattern, inhibiting the formation of the aluminum sidewall protection film during etching, and making anisotropic etching of aluminum difficult.